As a rear seat of a vehicle, one is known in which a tiltable unit adapted to be tilted to be used as an armrest or a table is provided in a middle seatback region of a three-seater seat (see JP-U-H04-23454). When not used, the tiltable unit is accommodated into a receiving recess formed in the seatback and thus is maintained in a state of forming a back resting surface flushed with the seatback. Also, by pulling forward a grip attached to the tiltable unit, the tiltable unit is adapted to be tilted from the accommodated state to a spread state where the tiltable unit can be used as an armrest or a table. In addition, a stopper structure is provided between the receiving recess of the seatback and the tiltable unit, so that the tiltable unit can be kept locked in the receiving recess without coming out from the receiving recess when the vehicle is suddenly braked or when the seatback is erected from a forward tilted position, or the like. The stopper structure is adapted to be released from such a locked state by an operation of pulling forward the grip attached to the tiltable unit.